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  • #16
    To be clear i have six strings total, idk if that makes a difference but that is all the least of my problems. Any word on how to turn a 2awg wire into 3x8awg wires relatively cheaply and without breaking code? Can I use a tap (T, power distribution block?) and not have circuit protection if the 8 awg wire is rated for 40 ish amps and the inverter can only output 32 amps?

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    • #17
      Originally posted by Markyrocks69 View Post
      To be clear i have six strings total, idk if that makes a difference but that is all the least of my problems. Any word on how to turn a 2awg wire into 3x8awg wires relatively cheaply and without breaking code? Can I use a tap (T, power distribution block?) and not have circuit protection if the 8 awg wire is rated for 40 ish amps and the inverter can only output 32 amps?
      I think your best option if you want to keep the 2 7.7s that you got is running new wire...

      I recently installed a somewhat similar system. (66) 345w panels in 6 strings to (2) 7.7-40s. #10 wire is more than sufficient. The systems should run a little more efficiently if each string is independent. You essentially get string level optimization.

      I would pair up the 60 panels into strings of 5. Each parallel string of 5+5 (10) can run on #10 wire to its own MPPT on the SMA. So basically 6 strings of 10 panels with each string being two strings of 5 in parallel. It sucks to rerun wire but IMO this would give you the best result....
      Last edited by nwdiver; 06-19-2019, 01:48 AM.

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      • #18
        Originally posted by nwdiver View Post

        I think your best option if you want to keep the 2 7.7s that you got is running new wire...

        I recently installed a somewhat similar system. (66) 345w panels in 6 strings to (2) 7.7-40s. #10 wire is more than sufficient. The systems should run a little more efficiently if each string is independent. You essentially get string level optimization.

        I would pair up the 60 panels into strings of 5. Each parallel string of 5+5 (10) can run on #10 wire to its own MPPT on the SMA. So basically 6 strings of 10 panels with each string being two strings of 5 in parallel. It sucks to rerun wire but IMO this would give you the best result....
        As I said b4 I'd rather move the inverter to the array and just use the existing wire as the ac side b4 I did that. I've done voltage drop calculations and it should be fine. I'd just basically have a box with a splice where the inverter was originally going to be.

        As far as the string configurations. I got the high efficiency Panels that have like anti shading technology or whatever. It doesn't look like a regular panel each panel has like hundreds of 1x 6 inch cells so I'm not too concerned about in efficiency in the strings. They're sunpower p17-340 blah blah pretty sick. Seem like very good output.
        Last edited by Markyrocks69; 06-19-2019, 02:11 AM.

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        • #19
          Originally posted by Markyrocks69 View Post

          As I said b4 I'd rather move the inverter to the array and just use the existing wire as the ac side b4 I did that. I've done voltage drop calculations and it should be fine. I'd just basically have a box with a splice where the inverter was originally going to be.
          I must have missed that post; What size wire is already run? If you mount the inverters at the array then it's fairly straight forward. I'm assuming you have X21-345s with Vmp of ~57v? 5 panel strings. If you pair them you don't need a combiner box. You'll have 6 pairs and each 10 panel set can have its own MPPT. 6 panel strings might be a little more efficient but then 1 inverter will have 3/5ths of the panels...

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          • #20
            Originally posted by nwdiver View Post

            I must have missed that post; What size wire is already run? If you mount the inverters at the array then it's fairly straight forward. I'm assuming you have X21-345s with Vmp of ~57v? 5 panel strings. If you pair them you don't need a combiner box. You'll have 6 pairs and each 10 panel set can have its own MPPT. 6 panel strings might be a little more efficient but then 1 inverter will have 3/5ths of the panels...
            no I don't have x series panels. I wish. It's the performance series p17 340 com
            https://www.google.com/url?sa=t&sour...Elb6ZgQnFStm9q

            the vmp is 42.5. The wire I have run from the array to the house is 2awg alluminum. Its roughly 150ft at the farthest point. I have 6 wires total. All 2 gauge use-2. So I'm just going to have to use the wire I designated as ground for my neutral. I was honestly thinking that wire is probably big enough to combine the neutrals into the one wire inside the house and then split them again at the array and I'd be able to use one wire as ground? I just don't think that will fly with the inspector bc they want that ground wire not to have any splices. I was thinking of putting in some rods at the array but I'd assume that they'd want those rods bonded with the ground rods for my main electrical service. That's why I'm thinking about biting the bullet and digging a trench from the main rods to the array and just running a new ground wire out there.

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            • #21
              Originally posted by Markyrocks69 View Post

              As I said b4 I'd rather move the inverter to the array and just use the existing wire as the ac side b4 I did that. I've done voltage drop calculations and it should be fine. I'd just basically have a box with a splice where the inverter was originally going to be.

              As far as the string configurations. I got the high efficiency Panels that have like anti shading technology or whatever. It doesn't look like a regular panel each panel has like hundreds of 1x 6 inch cells so I'm not too concerned about in efficiency in the strings. They're sunpower p17-340 blah blah pretty sick. Seem like very good output.
              I'm hoping you did look at shading effects since you are using a string inverter. The Sunpower P17-340 are set up to minimize shading in a commercial rooftop type setting, landscape orientation, since the cells are arranged in rows of series connected solar cells with lots of bypass diodes. What panels you have, how they are oriented and how shading tracks over your array can really make a difference in power output. Shading one entire panel in your string will definitely reduce your total output.

              I have two strings of 9 panels parallel connected to one MPP (because of SMA rapidshutdown) where one panel is shaded during December; power output is severely reduced. Not Zero output, but not 17 panels worth of power either. For what its worth.

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              • #22
                Originally posted by oregon_phil View Post

                I have two strings of 9 panels parallel connected to one MPP (because of SMA rapidshutdown) where one panel is shaded during December; power output is severely reduced.
                Yep; This is why I love having each string on an independent MPPT. I have a similar problem with an older SMA inverter. 3 strings on 1 MPP. When 3 of my back panels get shaded in the evening the entire array suffers. If I had a newer inverter with 3 MPPTs (one for each string) I'd only lose those 3 panels. My production decline would be ~7% instead of ~20%.

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                • #23
                  Originally posted by oregon_phil View Post

                  I'm hoping you did look at shading effects since you are using a string inverter. The Sunpower P17-340 are set up to minimize shading in a commercial rooftop type setting, landscape orientation, since the cells are arranged in rows of series connected solar cells with lots of bypass diodes. What panels you have, how they are oriented and how shading tracks over your array can really make a difference in power output. Shading one entire panel in your string will definitely reduce your total output.

                  I have two strings of 9 panels parallel connected to one MPP (because of SMA rapidshutdown) where one panel is shaded during December; power output is severely reduced. Not Zero output, but not 17 panels worth of power either. For what its worth.
                  All I can say about this is I'm in the best possible location on my property and as far as I can tell I get next to no shade 90% of daylight hours . This is honestly the least of my concerns atm. I appreciate your concerns. With my system being a ground mount array if I see any glaring negative effects over time it wouldn't be a huge ordeal to rearrange the strings.

                  My question is if say 2 sets of 5 are wired together in parallel doesn't the output get reduced to the lower of the 2 voltages? Like if 1 positive is 250 and the other is 125, doesn't the entire output drop to 125?

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                  • #24
                    Originally posted by Markyrocks69 View Post

                    All I can say about this is I'm in the best possible location on my property and as far as I can tell I get next to no shade 90% of daylight hours . This is honestly the least of my concerns atm. I appreciate your concerns. With my system being a ground mount array if I see any glaring negative effects over time it wouldn't be a huge ordeal to rearrange the strings.

                    My question is if say 2 sets of 5 are wired together in parallel doesn't the output get reduced to the lower of the 2 voltages? Like if 1 positive is 250 and the other is 125, doesn't the entire output drop to 125?
                    If you have 2 sets of 5 why would one have a lower voltage? I retract my previous recommendation of strings of 5 however... I thought you had higher voltage panels. If each panel has a Voc of ~50v I would do strings of 10 and run each string independently...

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                    • #25
                      Originally posted by nwdiver View Post

                      If you have 2 sets of 5 why would one have a lower voltage? I retract my previous recommendation of strings of 5 however... I thought you had higher voltage panels. If each panel has a Voc of ~50v I would do strings of 10 and run each string independently...
                      In a shading scenario, I thought that the point of smaller sets of strings wired in parallel was to avoid having a string of 10 output turn bad bc of one panel vs output of 5 going south if one panel shaded and having 5 do ok if you had 2 sets of 5 vs 1 string of 10. My plan is to have 6 strings of 10 panels, it should be 425v per string. But were way off topic

                      Is the idea of using a power distribution block bad? Is there any kinda reducer product?

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                      • #26
                        Originally posted by Markyrocks69 View Post

                        In a shading scenario, I thought that the point of smaller sets of strings wired in parallel was to avoid having a string of 10 output turn bad bc of one panel vs output of 5 going south if one panel shaded and having 5 do ok if you had 2 sets of 5 vs 1 string of 10. My plan is to have 6 strings of 10 panels, it should be 425v per string. But were way off topic

                        Is the idea of using a power distribution block bad? Is there any kinda reducer product?
                        No. A single string of 10 is better than parallel strings of 5. If 1 panel is shaded in a string of 10 the bypass diode shunts current and the other 9 panels are completely unaffected. If one panel is shaded in 2 combined strings of 5 each the voltage drops on both strings and all panels are affected. You effectively have a string of 4 paralleled with a string of 5 and that hurts the output of all panels.

                        If you have 6 strings of 10 panels then just run each string to its own MPPT since you have (2) inverters with 3 MPPTs each. Nothing else is needed. IMO the easiest plan is to just keep the inverters mounted at your home and run 6 pairs of #10 wire. That's all you would need. No distribution blocks or combiner boxes or fuses. Just run a pair of #10 wire from each string to each MPP channel.
                        Last edited by nwdiver; 06-20-2019, 03:59 PM.

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                        • #27
                          I know that does sound easy but I'd have to RIP out and throw away like 500$ worth of wire (it might have been more I cant remember) then buy like another....2000 ft of 10 gauge wire. That's going to be at least 500$. Then the labor. I just don't have it in me to go that route

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                          • #28
                            Originally posted by Markyrocks69 View Post
                            I know that does sound easy but I'd have to RIP out and throw away like 500$ worth of wire (it might have been more I cant remember) then buy like another....2000 ft of 10 gauge wire. That's going to be at least 500$. Then the labor. I just don't have it in me to go that route
                            I thought you were already considering digging a trench for a ground? I agree swapping wire sucks... but IMO it sucks less than relocating the inverters or trying to split the wire. And it would give you a more robust array. There's also the SPS option with the inverters that wouldn't be as effective if they're 150' way.

                            https://www.youtube.com/watch?v=Z9r8XfmErNs

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                            • #29
                              Originally posted by nwdiver View Post

                              I thought you were already considering digging a trench for a ground? I agree swapping wire sucks... but IMO it sucks less than relocating the inverters or trying to split the wire. And it would give you a more robust array. There's also the SPS option with the inverters that wouldn't be as effective if they're 150' way.

                              https://www.youtube.com/watch?v=Z9r8XfmErNs
                              That sps is not even a consideration. I have a emergency backup generator setup that supplies the whole house. I'm content with that as is.

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